Hi Sri,
I tried both situation, reversing at the same date on GR and some days later but the system didn't modify the points.
I've found a SAP article talking about that:
The last score recorded for a goods receipt is cancelled if:
The GR was unpunctual (GR date differs from statistical delivery date) and a reversal
document is then posted.
The GR was punctual (GR date same as statistical delivery date), the GR quantity less
the reversal quantity is below the minimum delivery percentage, and a reversal
document is then posted.
Based on this article, I understand that the system should cancell the points, but it isn't 
Link: Cancellation of Score in Case of GR Reversal - MM Vendor Evaluation - SAP Library